In recent years, a demand for improvement in fogging and fouling occurring on substrate surfaces such as a plastic surface and a glass surface has increased.
As a technique to solve this problem of fogging, an antifogging coating material obtained by adding a reactive surface active agent to an acrylic oligomer has been proposed, and a cured product film obtained from this antifogging coating material has been improved in hydrophilicity and water absorption properties (see, for example, non patent literature 1). As a technique to solve the problem of fouling, there has been noted an antifouling material having self-cleaning properties (antifouling properties) that hydrophilicity of a surface is improved to allow dart (hydrophobic substance in the outside air, etc.) having adhered to an outer wall or the like to come out by means of rain fall or showering of water, and thereby the dart is efficiently removed (see, for example, non patent literature 2 and non patent literature 3).
As typical resins having hydrophilicity, a large number of resins having many hydroxyl groups in a molecule, such as polyvinyl alcohol, are known. These resins having hydrophilicity exhibit different properties due to a difference in their chemical structures, and therefore, development of products suited for the properties have been made.
As other resins having hydrophilicity, there have been studied, for example, a polymer described in a patent literature 1, which is obtained by using 3-sulfopropyl methacrylate potassium salt, 2-acrylamido-2-methylpropanesulfonic acid sodium salt and polyethylene glycol diacrylate; a polymer described in a patent literature 2, which is obtained by using 3-sulfopropyl methacrylate sodium salt and long-chain urethane diacrylate (manufactured by Shin-Nakamura Chemical Co., Ltd., trade name “NK Oligo UA-W2A”); a polymer described in a patent literature 3, which is obtained by using 2-sulfoethyl acrylate, tetrahydrofurfuryl acrylate, trimethylolpropane acrylate and spiroglycol urethane diacrylate; a polymer described in a patent literature 4, which is obtained by using 2-sulfoethyl methacrylate and/or (meth)acrylate having phosphoric acid group and an epoxy resin; a polymer described in a non patent literature 4, which is obtained by using hydroxyethyl methacrylate, sufoalkylene (C6 to C10) methacrylate and methylene bisacrylamide; and a polymer described in a patent literature 8, which is obtained by using sulfonic acid-based (meth)acrylate, phosphoric acid-based (meth)acrylate, polyvalent (meth)acrylate and an ethanolamine-based compound.
In the patent literature 1, it is described that the resulting transparent gel can be used as a bio-adhesive. In the patent literature 2, it is described that the resulting polymer is excellent in ink absorption properties, has high water resistance and can be used as a recording material for use in an inkjet recording system free from blocking. In the patent literature 3, it is described that the resulting polymer can be used as an adhesive that firmly bonds a metal hub for driving an optical information disc to a resin substrate. In the patent literature 4, it is described that the polymer obtained by not only crossliking of an ethylenically unsaturated bond but also crosslinking of a sulfonic acid group or a phosphoric acid group through ionic reaction with an epoxy group can be used as an electrically conductive cured film having excellent mechanical performance, solvent resistance, film-forming properties, adhesion properties, transparency and abrasion resistance.
In the non patent literature 4, it is described that hydrophilicity of a slightly crosslinked film formed on a glass varies (in advancing contact angle and receding contact angle) with an alkylene chain length (C6 to 010) of sulfoalkylene methacrylate used as a monomer, and it also varies with hydration time.
The above polymers, however, have low degree of crosslinking between molecules and have high solubility in water, or absorb water and easily become gel-like though they are insoluble in water, or are liable to be damaged because their surfaces are soft, or have insufficient hydrophilicity, and therefore, they cannot be said to be satisfactory for use as antifogging materials, antifouling materials, etc.
In a patent literature 6, there has been proposed, as a technique to produce a hydrophilic molded product, a two-layer structure obtained by two-time coating comprising coating a surface of a substrate with a crosslinking polymerizable monomer composition, controlling ultraviolet irradiation dose to form a crosslinked polymer which has been incompletely polymerized, then coating a surface of the crosslinked polymer with a hydrophilic monomer and irradiating it with ultraviolet rays again to perform block or graft polymerization of the hydrophilic monomer onto the surface of the crosslinked polymer.
The above technique, however, is apparently complicated and costs high as compared with a general technique of a single-layer structure obtained by one-time coating comprising coating with a composition comprising a hydrophilic monomer and a crosslinking monomer and polymerizing the composition by means of ultraviolet rays or the like, and this technique is liable to impair smoothness of the surface, so that this technique cannot be said to be a preferred technique.
In the patent literature 8, there has been proposed a technique wherein a coating agent, which has been improved in solubility in organic monomers such as polyvalent (meth)acrylate and phosphoric acid-based (meth)acrylate by adding an ethanolamine-based compound to sulfonic acid-based (meth)acrylate having low solubility in organic solvents, is applied and subjected to UV irradiation to form a coating film having excellent antifogging properties and smoothness.
In this technique of the patent literature 8, a two-layer film consisting of a resin layer and a liquid layer is formed on the resulting substrate, but this two-layer film has tack. When this two-layer film is evaluated as such, it is transparent, has high antifogging properties and high smoothness and is hydrophilic. In this two-layer film, however, the upper layer (liquid layer) easily dissolves out by water washing or the like and vanishes. The film mainly composed of the residual resin layer has no tack, but the hydrophilicity is markedly lowered and the antifogging properties disappear.
Also the present inventors have previously proposed, as a technique to solve the above problems, a polymer using a hydroxyl group-containing (meth)acrylamide compound (patent literature 5).
In general, high surface hardness and high hydrophilicity can be mentioned as properties required for resins for antifouling coating. As properties required for resins for antifogging coating, relatively high surface hardness and hydrophilicity higher than that required for antifouling coating can be mentioned.
As a proposal to satisfy all of these requirements and to completely overcome the problems of “fogging” and “fouling”, a proposal of a single-layer film in which anionic hydrophilic groups are inclined to (concentrated at) the surface has been made (patent literature 7). The hydrophilic film obtained by this invention is transparent, has extremely high hydrophilicity, is excellent in antifogging properties, antifouling properties, antistatic properties, quick drying properties (high drying rate of adhesive moisture) and chemical resistance, is hard and also has excellent scratch resistance.
This single-layer film, however, is lowered in transparency, is lowered in hydrophilicity and suffers cracking in some cases depending upon the production conditions.
Therefore, there is yet room for improvement in the above single-layer film.